The assignee of this invention has pioneered the development and application of methods and apparatus for foaming polymeric materials such as hot melt thermoplastic adhesives, polymeric coatings, paints and other thermoplastic and/or thermosetting materials. In the case of hot melt adhesives, for example, it has been discovered that the strength of an adhesive bond achieved with a given volume of selected hot melt adhesive can be appreciably improved if the adhesive is applied as a foam rather than as a conventional non-foamed adhesive.
As disclosed, for example, in U.S. Pat. Nos. 4,059,466 and 4,059,714 to Scholl et al, initial attempts at "foaming" polymeric materials involved the mixture of hot melt thermoplastic adhesive and a blowing agent wherein a polymer/gas adhesive solution is formed and transmitted under pressure to and through a dispenser. Upon emerging from the nozzle of the dispenser into atmospheric pressure, the gas evolves from the solution in the form of small bubbles causing the adhesive to expand volumetrically. The resultant adhesive in an uncompressed state sets up as a homogeneous solid foam having gas cells substantially evenly distributed throughout the adhesive.
This technology has been extended to polymeric materials having a much higher viscosity than hot melt thermoplastic adhesives, as disclosed in U.S. Pat. No. 4,778,631 to Cobbs, Jr., which is owned by the assignee of this invention. Whereas hot melt adhesives have a viscosity typically in the range of about 2,200 cps to 20,000-35,000 cps, "high" viscosity polymeric material such as thermoset materials used as adhesives, seals and gasketing material have viscosities in the range of about 50,000 cps to about 1,000,000 cps. As disclosed in the U.S. Pat. No. 4,778,631 to Cobbs, Jr. et al, in order to form a solution of "high" viscosity polymeric material and gas, a low energy input disc mixer is employed to force the gas into solution with the polymeric material. When the solution is released to atmosphere, a homogeneous foam is formed wherein the gas is released from solution and becomes entrapped in the polymer.
It has been found that a number of practical problems are created in the application of high viscosity, foamed polymeric materials in a production environment. Because of the extremely high viscosity of such polymeric materials, dispensing systems must "dead end" at the dispenser, i.e., the material cannot be efficiently recirculated from the source, through the dispenser and back to the source because of the excessive amount of energy needed to move such material and the bulk of the conduit required. If the high viscosity material is dead-ended or stopped within the dispenser, unsatisfactory variations in the amount of material discharged from the dispenser can occur, particularly when the dispenser is operated intermittently. Each opening and closing of the valves associated with the dispenser to obtain intermittent discharge of material can result in pump surges and pressure fluctuations which, in turn, cause wide flow variations of the polymeric material discharged from the dispensers. Excessive application of material can result in a messy or sloppy appearance where the polymer is applied as a seam sealer, and/or can result in the formation of a gasket having an uneven profile. Wide pressure fluctuations also can result in the premature formation of foam within the dispenser if the pressure falls below the level required to maintain the gas in solution, or the formation of poor quality foam if the pressure is too high within the dispenser and the gas is not readily permitted to leave solution.
The problems associated with intermittent application of high viscosity polymeric materials have been addressed to some extent in the prior art. For example, in U.S. Pat. No. 4,922,852 to Price, owned by the assignee of this invention, a dispenser is provided with a servo-actuator including an electro-pneumatic servo-valve which operates a double-acting piston actuator for a variable fluid metering valve. A pressure sensor at the nozzle of the dispenser generates a pressure signal which is correlated to the instantaneous flow rate of the dispensed fluid to obtain precise control of the amount of fluid dispensed even when the dispenser is operated intermittently. While the system disclosed in the U.S. Pat. No. 4,922,852 operates effectively, it is relatively expensive because of the rather sophisticated control systems which are required to monitor and control the fluid flow.
Another problem with many systems of the type disclosed in the U.S. Pat. No. 4,922,852 is that they are not designed for use with "foamed" polymeric materials, i.e., a pressurized polymer/gas solution such as a highly viscous polymeric adhesive, sealant and/or gasketing material in which gas is contained in solution with such material. In order for a system to effectively dispense foamed polymeric material, the gas must remain in solution throughout its passage to and through the dispenser. The apparatus disclosed in U.S. Pat. No. 4,922,852, for example, employs a dispenser having a relatively large cavity between the discharge outlet of the dispenser and its flow control valve within which the adhesive could be permitted to foam when the valve is closed creating a potential problem of drool or leakage of the foamed adhesive out of the cavity. Apparatus of this type may be unacceptable for use in connection with pressurized solutions of polymeric material and gas.
It has been observed that another practical problem often occurs in the course of dispensing highly viscoelastic polymeric materials such as adhesives, sealants and gasketing materials, particularly in automated applications wherein a robot arm or the like manipulates a dispenser in a non-linear flow path with respect to a substrate. In the application of a gasketing material onto a substrate, for example, a dispenser typically discharges material at a constant flow rate while the robot arm moves in an essentially straight path with respect to the substrate. When the dispenser is moved in a non-linear path, however, such as around a corner, the highly viscoelastic polymeric material tends to be pulled away from the corner if the dispenser continues to operate at the same flow rate. There has therefore been a need to provide automatically manipulated dispensers with the capability of varying the flow rate at which polymeric material is dispensed therefrom, without substantially adding to the expense of the dispenser and/or the controllers associated with the dispenser.